Wendy Gu1 Mehrdad Kiani1

1, Stanford University, Stanford, California, United States

Bimetallic interfaces are ubiquitous features in metal alloys, composites and machines. It remains challenging to design bimetallic interfaces with high strength and damage tolerance because of the diversity of interfacial features (e.g. atomic bonding, geometry, chemical composition) and their competing mechanical effects and complex interactions. In particular, lattice mismatch between the two metals determines whether the interface has perfect atomic registry (coherent interface) or contains defects such as misfit dislocations (semicoherent interface). The effect of atomic coherency on interfacial strength and plasticity is well established for arrays of randomly dispersed precipitates in metallic alloys, but this knowledge cannot be readily applied to nanostructured multilayers or nanostructures due to differences in geometry, dislocation density and proximity to other interfaces. To address this issue, it is necessary to nondestructively image coherent versus semicoherent bimetallic interfaces in nanostructures, and quantify their mechanical response.
Here, we study deformation at individual bimetallic interfaces by compressing colloidal core-shell nanocubes inside of a scanning electron microscope. Bimetallic nanocubes are ~50 nm in size. Interfacial coherency is manipulated by varying lattice mismatch at the core-shell interface. We have synthesized Au@Ag nanocubes with a lattice mismatch of 0.2%, Au@Cu nanocubes with a lattice mismatch of 12%, and monometallic Ag nanocubes. Transmission electron microscopy and X-ray diffraction are used to characterize nanocube structure and composition. We find that Au@Ag nanocubes have low lattice strain, and a coherent core-shell interface. The Au@Cu nanocubes have higher strain, and a semicoherent interface that contains dislocations. Nanocubes are compressed at 0.1 s-1 under load control. The strength of the Au@Ag nanocubes is found to be ~800 MPa. The Au@Ag stress-strain curve is serrated with large, discrete stress drops, similar to the stress-strain behavior of single crystal metallic nanostructures. This indicates that dislocations cut across the coherent interface in Au@Ag nanocubes. The mechanical response of the core-shell nanocubes is compared to that of monometallic Ag nanocubes in order to differentiate the effect of the bimetallic interface and sample size.